According to the pilot, while en route to his destination airport, he decided to land on a sandbar in a nearby river. After touching down on the sandbar, the pilot realized that the available stopping distance was less than he had thought, so he applied hard braking in order to stop the airplane. During the braking sequence the airplane nosed over onto its back, resulting in the top portion of the rudder being bent over. The pilot said that he had not properly inspected the landing zone, and that his airspeed was too fast on final. He also stated that the airplane did not experience any mechanical malfunctions.